Kenyan police killed at least 33 people in the capital during a crackdown following elections in August, Amnesty International and Human Rights Watch said on Monday.
Excessive force by police against protesters and residents in strongholds of opposition leader Raila Odinga caused the deaths in Nairobi, the report said.
President Uhuru Kenyatta defeated Odinga in the Aug. 8 election and days of protests followed. The Supreme Court last month voided the election citing procedural irregularities and ordered a re-run, which is to be held on Oct. 26.
"Researchers found that although police behaved appropriately in some instances, in many others they shot or beat protesters to death."
The report is likely to bolster the case of Kenyan activists and rights groups who accuse police of brutality and extrajudicial killings but say few officers are charged and convictions are extremely rare.
Police spokesman Charles Owino did not immediately respond to a request for comment. Police have said only looters and thugs were killed or injured in the violence.
The report said a 9-year-old was shot dead while standing on a balcony and a woman who was eight months pregnant fainted from inhaling teargas and was trampled to death.
The parents of a six-month-old baby told Reuters during the violence their child was clubbed by police in her home and died from brain trauma at a hospital several days later.
Odinga withdrew last week from the re-run saying the vote would not be fair, leaving Kenyatta as the only candidate. The president said the election would proceed.
Political uncertainty has blunted growth in Kenya, a Western ally that has East Africa's richest economy.
For the past two weeks, police used tear gas to disperse opposition demonstrations held twice weekly in the country's three biggest cities. The protesters had been calling on the election board to make reforms to ensure a fair poll.
On Thursday the government banned demonstrations in the central business district of Nairobi, the coastal city of Mombasa and the western city of Kisumu.
A group of U.N. human rights experts called for the government's ban on protests to be listed and denounced a "pattern of police brutality" in response to recent demonstrations.
The report brings the nationwide number of killings by police after the Aug. 8 vote to more than 45. Human Rights Watch last month documented 12 killings by police in western Kenya, the main opposition stronghold.
The country receives substantial financial support for its security services from the United States, Britain and other international donors.